A trickle of defections has Democratic House leaders wondering how long they can hold off calls for an investigation into the PMA Group and its ties to Pennsylvania Rep. John P. Murtha.

Rep. Jeff Flake (R-Ariz.) got only 17 Democratic votes when he introduced a privileged resolution in February calling for an ethics investigation into “the relationship between earmark requests already made by members and the source and timing of past campaign contributions.”

But Flake has kept trying — the sixth version of his resolution comes up for a vote this week — and he’s picked up support from eight Democrats who voted against his initial resolution.

And that has Democratic leaders worried.

“We are keeping our ear pretty close to the ground on this,” said a senior Democratic aide.

The aide noted that there has been “no groundswell of support” for Flake’s resolutions — and that Speaker Nancy Pelosi (D-Calif.) remains dead set against an investigation. Still, he said, House Democrats — who took power promising to “drain the swamp” in Washington — “may only be one bad story away from seeing some big break.”

“It’s a very slow trickle at this point, but that could change,” the aide said.

According to press reports, the FBI is investigating whether PMA founder Paul Magliocchetti, a former Appropriations Committee staffer, made “straw man” donations on behalf of his employees or other individuals to members of Congress, which is illegal under federal election law.

Following an FBI raid in November, the PMA Group dissolved, and Magliocchetti has retired and moved to Florida.

Flake introduced his first PMA resolution as Congress took up the $410 billion omnibus spending bill in February. The 17 Democrats who backed that initial resolution included a number of recent arrivals who could face tough reelection fights next year — Reps. Debbie Halvorson of Illinois, Ann Kirkpatrick of Arizona and Harry Teague of New Mexico.

Four similarly positioned Democrats joined them on the second vote: Alabama Rep. Bobby Bright, whose conservative district requires him to vote with the GOP on most controversial measures; Illinois Rep. Bill Foster, elected last year to the seat vacated in 2007 by former Republican Speaker Dennis Hastert; and two first-term Democrats who knocked off incumbent Republicans last fall — Idaho Rep. Walt Minnick and Florida Rep. Suzanne Kosmas, who made ethics a centerpiece of her campaign against then-GOP Rep. Tom Feeney.

Foster, a former scientist, read Flake’s initial resolution on the House floor before the vote and reasoned that it was too vague to establish a special ethics investigation, his spokeswoman said.

The first resolution failed to mention PMA by name but, rather, laid out a series of press reports about the firm following the FBI raid of its offices in November. In that initial resolution, Flake called for the ethics committee, or an investigative subpanel, to look for any connections — i.e., campaign contributions or lobbyist ties — between the recipients of earmarks and the lawmakers who requested them. He gave the ethics panel two months to complete such a wide-ranging probe.

“[Foster] just thought the first resolution was too broad,” said his spokeswoman, Shannon O’Brien.

Foster backed the second resolution — and each one to follow — because it narrowed the proposed investigation to PMA and its clients.

Indeed, the second resolution called on the ethics panel to investigate the timing of campaign contributions from clients and employees of “the raided firm” to the lawmakers requesting their projects, singling out PMA without actually referring to the now-defunct lobbying group by name.

Indiana Rep. Pete Visclosky, a senior member of the Appropriations Committee, joined the younger Democratic defectors in round two — despite the fact that his former chief of staff worked as a lobbyist with PMA and that the thousands of dollars he’s received in campaign contributions from PMA employees and clients over the years could make him a subject of the probe. Visclosky’s office did not return requests for a comment.

Then, last week, South Dakota Rep. Stephanie Herseth Sandlin and Indiana Rep. Baron Hill — co-chairmen of the fiscally conservative Blue Dogs — sided with Flake and other Republicans to call for a vote on the resolution. Days earlier, Hill had signaled his unease by voting “present” on another version of Flake’s privileged resolution. They were joined on this most recent vote by Washington Rep. Adam Smith, who runs the New Dems’ political action committee.

All eight Democratic offices were asked about their bosses’ change of heart, but only Foster’s offered an on-the-record explanation for his votes.

House Democrats built their new majority in large part by running hard against the “culture of corruption” in Washington. The Flake resolutions — coupled with revelations about Ways and Means Committee Chairman Charles B. Rangel (D-N.Y.) — have given Republicans a chance to regain some footing on the issue.

But the iconoclastic Arizonan — who has battled his own leaders on earmarks — insists that he’s not driven by hopes of partisan gain. He says his main goal is to root out corruption in the annual spending process.

“This is not a partisan issue,” Flake told POLITICO last week, “and I think most members recognize that on both sides of the aisle.”

Flake, a Republican anti-earmark crusader, promises to keep offering these resolutions until his colleagues give in. “This is a matter that has to be dealt with,” Flake said. “And if that means I need to keep offering it, I will.”

Privately, Democrats say that if Flake were serious about wanting an ethics committee investigation, he could file an ethics complaint against Murtha or other members tied to PMA. They note that Flake has failed to do that so far — and they say it’s because he knows Democrats could retaliate by filing their own ethics complaints against Reps. Jerry Lewis (R-Calif.) or Don Young (R-Alaska).